A temperature measuring transducer with such a measuring device is for example known from U.S. Pat. No. 5,317,520. The measurement of a temperature as a chemical or physical variable is undertaken with an electrical resistor, which alters its value depending on this variable. This resistor is connected in a three-wire or four-wire circuit to an evaluation device. Each side of the resistor has two lines for making contact between it and a four-wire circuit. Two of the lines are used to conduct a current through the electrical measuring resistor. The two other lines are used to tap off the voltage which is dropping at the measuring resistor during the measurement. This enables the value of the measuring resistor to be measured without a current flowing through the two lines via which the voltage is tapped off. This means that the measuring result is not influenced by a voltage drop at the measuring lines. In a three-wire circuit also described, three lines are used to connect the electrical measuring resistor to the evaluation device. In this case it is also possible to compensate for the influence of the line resistance on the measuring result, for example in that the voltage difference between a line, with which the voltage is tapped off at the resistor and current-carrying return line is determined and taken into account in a suitable manner in the calculation of the measuring result. The evaluation device of the known measuring transducer can test the lines for wire breaks by individually disconnecting each of the lines to the electrical resistor. A variation in the measured value can be detected in this test mode and it can thus be concluded that there has been a wire break. It is thus possible, using the known measuring transducer, to detect a break in a measuring line and to notify this error to a control center in an automation technology system. The measuring transducer switches to an error mode and not longer outputs temperature signals. Depending on the safety concept of the installation, this can have the disadvantage of triggering an immediate shutdown of the installation. After the error state has been notified to the operating personnel the defect in the line can be rectified by the maintenance personnel.
Since fault tracing in a large installation can be thoroughly time-consuming and longer shutdowns are to be avoided, measuring transducers, as known for example from the Abstract JP 2001-208617 A are configured as redundant devices to provide higher availability. With this type of redundant configuration, if there is a defect in a measuring transducer, another transducer can continue to be used for recording a process temperature, without the installation having to be taken out of operation after the defect. The disadvantage of a redundant arrangement at measuring transducers however is that it entails considerable expense since, in addition to the actual measuring transducers required for the automation of an installation, additional transducers must be disposed of which can take over the function of a failed measuring transducer where necessary.